The composition of even the more common rock types is highly variable. For example, sandstones may be bonded with silica or calcite, or otherwise cemented; or the quartz content in granite may vary by a factor of three or more. In addition, in-situ rock is affected by geological actions such as faulting or jointing that (usually) are followed by chemical processes that produce alteration and decomposition. Unless mechanical property tests are conducted at a scale such that the test specimen includes these defects in normal proportion, the results will not be representative of the in-situ rock. The specimen size that satisfies this requirement is generally too large to be tested in the laboratory because of the physical limitations of test equipment. The alternative is to test in-situ. This procedure is limited by difficulties encountered in preparing an area (specimen) of a shape such that the test results will be interpretable and also in applying a force of sufficient magnitude to an area of this size.
As a consequence of these limitations, virtually no laboratory tests have been made on specimens large enough to contain defects of geological origin, and only a few in-situ tests have been made under conditions that permit a satisfactory interpretation.
In the majority of existing techniques, attempts are made to measure the "instantaneous" displacement directly. A good example of this is the 'overcoring" method. In this procedure, a wellbore is drilled to the horizon to be tested. At this point, a smaller hole is cored in the bottom of the wellbore and either a displacement or stress measuring tool is seated in the corehole. A larger core barrel is then used to cut around the rock containing the seated tool while it is operating. As the large core barrel cuts into the formation, the rock inside the barrel is released from the formation and displaces itself "instantaneously" with respect to the measurement tool.
Such techniques are not only difficult to perform, but are also time consuming. Stringent requirements on the various operational procedures tend to limit measurement depths to a few hundred feet. Most of these devices are limited to use in mines, tunnels, quarries, and other types of excavation sites.
Therefore, what is needed is a method and tool to perform in-situ creep measurements in a formation immediately after it has been drilled or cored and which can be employed at depths exceeding a few hundred feet.